When using materials which are susceptible to moisture, it is desirable to know whether those materials have been exposed or are being exposed to a particular level of humidity of the surrounding atmosphere. Humidity indicators are commonly applied to materials and objects to indicate the relative humidity levels in the surrounding air. In desiccants or desiccant containing articles, humidity indicators are known to be applied to the desiccant itself or to the container to indicate the saturation level of the desiccant. Known desiccant containers are made from non-woven fabrics, metal, paper and plastics. When the level of moisture absorbed by the desiccant in the container reaches a specific level of saturation, the desiccant in the container has to be replaced or regenerated. Known containers have a patterned design which changes color depending on the moisture level in order for the user to determine whether the maximum limit has been reached.
Known desiccants and desiccant containers have humidity indicating compositions based on cobalt dichloride or other metal salts. However, these compositions contain considerable amounts of heavy metals and halide anions, which are undesirable in view of the environmental considerations.
For example, EP2085132 discloses a regenerative dehumidifying bag comprising a water vapour-permeable pouch (1), a desiccant material (2) filled in the pouch (1) and a colour-changeable humidity indicator (3) printed on an outer surface of the pouch (1). The humidity indicator (3) includes a base figure (31) of blue colour printed on an outer surface of the pouch (1) and a colour-changeable marking (32) formed as a mark disposed within the base figure (31) of the humidity indicator (3). The colour-changeable marking (32) includes a colour-changeable ink comprising cobalt chloride. Upon saturation of moisture absorbed by the desiccant material (2) in the pouch (1), the marking (32) with the cobalt chloride changes colour from a blue colour to a pink colour so as to be distinguishable from the blue colour of the base figure (31), indicating that the desiccant material (2) should be replaced.
EP2339340 discloses a humidity indicating composition containing no halogen compounds and no heavy metals. In particular, EP2339340 discloses a humidity indicating composition comprising (a) an organic pH indicator dye compound, (b) a Bronsted acid or base, (c) a polyol having the general formula HOCH—(CH2)n-CHOH, wherein n can be 0, 1, 2, 3 or 4, and (d) a solvent; wherein said components (a), (b), (c) and (d) do not contain halogen and/or heavy metals. The composition is used for preparing a humidity indicator card by applying the composition onto a sheet substrate and allowing the solvent (d) to evaporate. The evaporation of the solvent requires an extra step.
Other problems of existing indicator compositions are that they degrade and lose their indicating function over time due to exposure to condensation and steam during the regeneration process. This washes out the dye compounds and reduces product life.
There is a need in the industry for a humidity indicating composition which can be used for making various types of humidity indicating article.